This invention relates to a method of using achiral pulsed laser irradiation to enhance the amount of a particular chiral enantiomer in an enantiomeric mixture. More particularly, the invention relies upon controllable quantum interference effects generated in the system by multi-photon processes to alter the ratio of chiral enantiomers.
Chemical reactions which begin with achiral molecules in an achiral environment are incapable of producing reaction products which favor production of one enantiomeric product, e.g. L, over its mirror image D. In addition, the physical and chemical properties of two chiral enantiomers are similar, preventing the enhanced production of L over D by any direct chemical or physical method which relies upon achiral ingredients. However, the desire to produce a specific chiral enantiomer in preference to its mirror image, called asymmetric synthesis, is a longstanding challenge in synthetic chemistry. In particular, there is a great deal of interest in doing so for the production of stereospecific pharmaceuticals.
A number of methods are available to perform asymmetric synthesis. However, all rely upon the introduction, in the chemical reaction, of some chiral component (e.g. a chiral reagent, chiral enzyme or chiral template). By sharp contrast, the method proposed here requires neither an overall chiral initial molecular system nor chiral laser excitation. Rather, it relies upon the fact that the pulsed laser excitation of the D and L systems to the species in the excited electronic state can prepare a superposition of vibrational-rotational states which no longer possess the reflection symmetry of the excited molecular species. This superposition state differs if it is prepared from D or from L, allowing a laser-based method of differentiating between L and D and of enhancing the concentration of one enantiomer over another.
We note that alternative ideas, based on laser excitation, have been advanced. These methods all rely upon using chiral light (e.g. circularly polarized light) where the effects rely on high order perturbation theory, tend to be weak and hence without practical utility. We further note that our approach are peripherally related to established optical pumping scenarios studied in Quantum Optics. However, none of the Quantum Optics work focuses on the issue of enhancing the enantiomeric ratio nor does it provide a method for altering enantiomeric ratios.
Finally, note that is past work we considered the photodissociation of a molecule LAD, where A is an atom or molecule, to produce a controlled ratio of LA+D vs L+AD. In that case the excitation was to a set of continuum energy levels from a bound achiral molecule LAD, quite different from the bound-bound (from the bound chiral enantiomers L and D to a bound excited species) transitions invoked here. Further, out past work relied upon small non-Franck-Condon components of the dipole operator, required a means (e.g. external magnetic fields) of separating out product states with well defined projections of the total angular momentum along a space fixed axis, required an initial step to synthesize the particular molecule LAD and L an D. Hence, our past work is totally different in both concept and execution from the present invention described herein.
It is the object of this invention to provide a novel process for enhancing the enantiomeric excess of one chiral enantiomer over another in a mixture of chiral enantiomers.
The present invention is a laser-based method of enhancing the enantiomeric excess of one chiral enantiomer in a mixture of chiral enantiomers, denoted L and D (and related to one another by the inversion operation I). The molecule L and D is chosen so that electronic excitation is to an electronically excited species with stationary rotational-vibrational states which are individually either symmetric or anti-symmetric with respect to I. The mixture is irradiated with a series of achiral pulses of coherent laser light. By varying the frequencies, timing, and durations of these pulses one can selectively increase the enantiomeric excess of either L or D in the ground electronic state. In an alternate application, a molecule B is added to a mixture of L and D where B is chosen so that (a) L-B and B-D are stable geometrically distinct species on the ground electronic potential energy surface which are related to one another by inversion, and (b) LB and DB molecules are stable, achiral molecules in an excited electronic state such that LB and BD are either identical or interchange rapidly with one another. The mixture is irradiated with a series of achiral pulses of coherent laser light. By varying the frequencies, timing, and durations of these pulses one can selectively increase the enantiomeric excess of either LB of BD in the ground electronic state. Subsequent traditional chemistry strips the B to yield an excess of either L or D.
The advantages of this invention over previous approaches is considerable. First, only a minimal amount of chemistry is required to enhance the desired enantiomer whereas other asymmetric synthesis schemes require large numbers of complex chemical steps. Second, this process can serve as the final step in the chemical synthesis of a wide variety of molecules. That is, having synthesized a mixture of D and L compounds in traditional synthetic schemes, one can, assuming property molecular characteristics, add this process as an additional step to enhance the enantiomeric excess of the product. Hence this approach is applicable to the production of chiral enantiomers of a wide variety of molecules.